Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear topcoat to the base coat have become conventional as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image, and the clear coat is particularly important for these properties.
The clear coat compositions used in the automotive industry are generally one-component or two-component compositions. Currently, the automotive industry mostly utilizes one-component clear coat compositions. The most commonly used one-component compositions are aminoplast-cured.
Aminoplast-cured coatings such as polymeric polyol-aminoplast systems are known to provide many excellent coating properties. They are inexpensive, durable, and attractive. However, it is widely recognized that such coatings, particularly clear coats, have poor resistance to etching by atmospheric acidic pollutants. Because many geographic areas encounter acidic precipitation, acid etch resistance in coatings is becoming an increasingly desirable property, particularly for automotive coatings. Polymeric polyol-aminoplast coating systems of the prior art are not highly effective for providing protection against etching caused by acid rain.
Two-component clear coat compositions comprising polyols such as polyester polyols, polyurethane polyols, and acrylic polyols cured with polyisocyanates yield coatings with outstanding gloss and distinctness of image. However, the isocyanates are difficult to handle. They are sensitive to moisture and require cumbersome safety precautions because of their toxicity.
Coating systems of the prior art which are known to be durable include polymers having cyclohexyl and hydroxyalkyl functionality. Examples include vinyl fluoropolymers such as those described in U.S. Pat. No. 4,345,057 and acrylic polymers such as those disclosed in U.S. Pat. No. 4,565,730. The fluoropolymers of U.S. Pat. No. 4,345,057 are the reaction products of fluoroolefin monomers, cyclohexyl vinyl ether monomers, alkyl vinyl ether monomers, and hydroxyalkyl vinyl ether monomers. Unfortunately, the fluoroolefin raw materials needed for these fluoropolymers are very expensive. U.S. Pat. No. 4,565,730 discloses the use of acrylic polymers which may contain cyclohexyl(meth)acrylate and/or hydroxybutyl(meth) acrylate in coating compositions. These polymers are cured with polyisocyanates in a two-component composition. The polyisocyanates are also expensive and the handling and toxicity of the isocyanates are additional drawbacks. Like the fluoropolymer coatings, the coatings of U.S. Pat. No. 4,565,730 exhibit durability, but there is no teaching that either the fluoropolymers or the polyisocyanate cured systems are resistant to acid etching.
It has been discovered, surprisingly, that acid etch resistant coatings can be prepared from polymers containing cyclohexyl (meth)acrylate and certain hydroxyalkyl(meth)acrylate which are crosslinked with aminoplast crosslinking agents. The present invention provides a color-plus-clear coating system which avoids the problems of the prior art by providing improved acid etch resistance properties using less expensive, relatively non-toxic raw materials.